Correction of age-related optical changes in the eye becomes increasingly important as the life expectancy continues to increase. One age-related optical change in the eye is presbyopia, where people have difficulty in focusing close objects onto the retina because of decreased lens flexibility. Presbyopia usually begins to affect people in their forties, so there is a significant need for this vision correction. Ophthalmic lenses with fixed focusing properties have been widely used as spectacles and contact lenses to correct presbyopia and other conditions.
Ophthalmic lenses are most useful if they have adjustable focusing power (i.e., the focusing power is not static). Adjustable focusing power provides the eye with an external accommodation to bring objects of interest at different distances into focus. Adjustable focusing power can be achieved using a mechanical zoom lens. However, the mechanical approach makes the spectacle bulky and costly.
Different optical techniques have been exploited in bifocal lenses to allow both near and distance vision. For example, the user may have lenses providing different focusing power to each eye, one for near objects and the other for distant objects. Alternatively, by use of area division of the lens, bifocal diffractive lens or other division techniques, both near and distant objects are imaged onto the retina simultaneously and the brain distinguishes the images. Except for the bifocal diffractive lens, the field of view using these optical techniques is small. Furthermore, these optical techniques do not work well when the pupil is small, since the iris blocks the beam that passes through the annular portion of the lens. Another option for correction is the use of monovision lenses, where different focusing power is provided to each eye, one for near objects and the other for distant objects. However, the binocular depth perception is affected when monovision lenses are used.
Electrically switchable lenses (for example lenses having a layer of liquid crystal sandwiched between two conductive plates where the orientation of the liquid crystal changes upon application of an electric field) have been described for use in optical systems (see, for example, Kowel, Appl. Opt. 23(16), 2774-2777 (1984); Dance, Laser Focus World 28, 34 (1992)). In electrically switchable lenses, various electrode configurations have been studied, including Fresnel zone plate electrode structures (Williams, SPIE Current Developments in Optical Engineering and Commercial Optics, 1168, 352-357 (1989); McOwan, Optics Communications 103, 189-193 (1993)). Variable focal length liquid crystal lenses have been described (Sato, Jap. J. Appl. Phys. 24(8), L626-L628 (1985)). However the use of liquid crystal lenses in spectacle lens applications is limited due to many factors, including low diffraction efficiency when the focal length is changed and slow switching times resulting from the required thickness of the liquid crystal layer. An improved lens with adjustable focusing power is needed.